The use of gas plasma for processing semiconductor wafers is common in the art. For example, various techniques are described in J. Hollahan and A. Bell, Techniques and Applications of Plasma Chemistry, Ch. 9 (1974).
Semiconductor components are fabricated on a semiconductive substrate or wafer. The material of the wafer is generally silicon. In manufacturing semiconductor devices, a photosensitive polymer, generally referred to as a photoresist, is used. After selective exposure to optical radiation and subsequent chemical development, the photoresist hardens where it has not been removed and protects the underlying wafer from other chemicals. One method of removing photoresist from wafers after it has served its protective function is by using a gas plasma.
In general, the gas plasma used in removing photoresist is oxygen. More particularly, diatomic oxygen is first exposed to an electric field which transforms some of the diatomic oxygen into an oxygen plasma that contains some monoatomic oxygen, generally referred to as atomic oxygen. Atomic oxygen is capable of reacting with the photoresist by breaking its polymer chains such that the photoresist is removed from the semiconductor wafer by the combined action of the atomic oxygen and the molecular oxygen. The resultant by-products are gases such as H.sub.2 O, CO and CO.sub.2.
Prior art oxygen plasma reactors for removing photoresist, an example of which is shown in FIG. 2A, consist of a cylindrical quartz reactor. A plurality of semiconductor wafers, each of which has a layer of photoresist on its surfaces, are positioned within the reactor. Metal electrodes are positioned around the reactor, one of which is connected to a radio-frequency (RF) generator operating at 13.56 MHz or some harmonic of that frequency and the other is connected to the ground. The quartz reactor also includes a gas input manifold and an exhaust manifold.
Other prior art plasma reactors, not shown, include single-chamber reactor that has an electrode within the chamber, as best exemplified in U.S. Pat. No. 4,230,515. In addition, prior art reactors include double-chamber reactor in which the plasma is generated in one chamber and the work such as photoresist removal is performed in a second chamber. The plasma may be transported between the two chambers either through a narrow channel or through narrow tubes. The primary disadvantage of the double-chamber reactor is the likelihood of plasma degeneration before it could perform the removal of the photoresist, that is, atomic oxygen tends to recombine to diatomic oxygen on the walls of the channel or tubes.
In prior single chamber reactors with external electrodes, the electrodes are wrapped around the entire sides of the cylindrical reactor so that the electric field fills the whole volume of the reactor. However, due to the electrical skin effect of the RF discharge, the electric current produced tends to "hug" the reactor wall. This effect is analogous to the phenomena of high frequency current flowing near the surface or skin of a metal conductor. Thus, most of the atomic oxygen is produced near the walls of the reactor and is pumped out of the reactor without getting near the wafers. The only atomic oxygen that is involved with the removal process is that which diffused into the center of the reactor where the wafers are placed and then diffusing between the wafers.